A voltage comparator may be used to compare the value of an input voltage signal VIN with a reference voltage VREF. In this context, the voltage comparator may be viewed as an amplifier to amplify the difference VREF−VIN with high gain. Such voltage comparisons are often performed in analog-to-digital converters, peak detectors, zero-crossing detectors, and full-wave rectifiers. Some or all of these mentioned circuits may be utilized in a computer system. For example, consider the computer system illustrated in FIG. 1. Microprocessor die 102 comprises many sub-blocks, such as arithmetic logic unit (ALU) 104 and on-die cache 106. Microprocessor 102 may also communicate to other levels of cache, such as off-die cache 108. Higher memory hierarchy levels, such as system memory 110, are accessed via host bus 112 and chipset 114. In addition, other off-die functional units, such as graphics accelerator 116 and network interface controller (NIC) 118, to name just a few, may communicate with microprocessor 102 via appropriate busses or ports. Power supply 120 provides a regulated voltage to microprocessor 102, as well as perhaps other system components in FIG. 1. Voltage comparators, for example, may find application in power supply 120 as part of a full-wave rectifier, or may find application in NIC 118 as part of an analog-to-digital converter for signal communication over a physical link (not shown).
A prior art voltage comparator is shown in FIG. 2. The voltage comparator of FIG. 2 is seen to be a differential amplifier comprising differential pair 202 and 204, current source 206, and a current mirror (transistors 208 and 210). Two voltage gains may be defined for the differential amplifier of FIG. 2. Let AIN denote the input voltage gain |vOUT/vIN| when VREF is held constant, where vOUT is the small-signal voltage component of VOUT and vIN is the small-signal voltage component of VIN. Let AREF denote the reference voltage gain |vOUT/vREF| when VIN is held constant, where vREF is the small-signal voltage component of VREF.
Prior art voltage comparators tend to have approximately equal 3 dB bandwidths for the two gains AIN and AREF. A typical frequency response for the comparator of FIG. 2 is shown in FIG. 3, where curves for the two voltage gains AREF and AIN are labeled as such. From FIG. 3, it is seen that the two 3 dB bandwidths are approximately equal to each other.